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Abstract
Delhi as the seat of power has been the focus of attention for administrators and planners. Its fast growing population, natural and manmade disaster proneness, changing security perceptions and pressure for maximum utilization of space call for exploring newer avenues for development. Sky scrappers as well as the sophisticated underground living and working space are the features of inevitable future.

Subsurface maps of Delhi, emerged as an outcome of a research project undertaken by Central Building Research Institute, are the essential prerequisite of further urban growth of the mega city. Whether it be new development of any uninhabited area, delineation of earthquake prone localities, building up a failsafe corridors creating strategically sensitive space, subsurface maps are one of the best required tools.

In this paper, authors discuss about the methodology used for preparation of subsurface maps as well as their results.

Introduction
Delhi being the capital, has always been the seat of power right from ancient times, except for some decades during the British period. Its strategic geographic location, as well as the typical geological geomorphological setting makes it so important. Its importance is to attract people, industrial and strategic establishments in great proportion, simultaneously converting the mega city into a time bomb being vulnerable to natural and manmade hazards. It has to expand with faster pace to cater the growing need, but preparedness to efficiently manage the said disasters should be seen as imminent urgency by the administrators and planners.

Delhi region falls in zone IV of seismic zoning map of India, which is located on the margin of Himalayan foredeep. The zone has fairly high Seismicity with general occurrence of earthquakes of 5-6, a few of magnitude 6-7 and occasional incidence of 7.5-8.0 magnitude shocks (RITES Report, 1996). The first recorded major earthquake in this region occurred on 15th July 1720 of intensity IX (Chandra, 1992; Srivastava & Somayajulu, 1996; Tandon, 1975). Subsequently other historical events occurred in 1803, 1819, 1905, 1934, 1937, 1945, 1949, 1958, 1960, 1966, 1975, 1980, 1994 during which intensities between VII and IX are believed to have been experienced as indicated by damage pattern. During recent times the area has been shocked by a number of earthquakes, out of these earthquake of 27th August 1960 with magnitude 6.0 and having epicentral tract between Delhi and Gurgaon was most significant. Recent Himalayan earthquake namely Uttarkashi and Chamoli were experienced in Delhi, and ultimate probability of major earthquakes in the region should make the authorities awake to the problem.

The earthquake history of Delhi region indicates fairly high Seismicity for the city of Delhi. The most active area of the region is considered to be the trijunction of the Delhi-Hardwar ridge, Lahore-Delhi ridge and axis of Delhi folding. Most of shocks are interpreted to be shallow focus and have concentrated around Sonepat, Rohtak and Gurgaon region in and around Delhi may be considered as seismically very active and the tectonic elements of the area are considered capable of generating an earthquake of magnitude 7 on Richter Scale (Chadha & Mathur, 1995).

Some areas of Delhi, due to their unique subsurface settings, are relatively more prone to damage in case of such eventualities. In such cases, apart from the density of population and type and quality of construction of buildings, thickness of sedimentary deposits play very critical role. This paper gives an account of CBRI's effort to map Delhi on the basis of thickness of soil cover, which may be very useful to the common people, Government and private builders, administrators, planners, rescue operation managers and many more sections of the society.

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